Process for carboxylation of naphthoic acid to naphthalene dicarboxylic acid

ABSTRACT

Disclosed is a method for selective carboxylation of naphthoic acid, or other aromatic mono-acids to form primarily 2,3-naphthalene dicarboxylic acid (2,3-NDA) or other aromatic diacids which comprises reacting said aromatic mono-acid in the presence of one or more metal oxide catalysts, alone, or in combination, and in the presence of about 0.2 to 0.8 moles excess of base over aromatic mono-acid, at a temperature of about 380° C. to about 420° C., and, in a second step, disproportionating the product of said selective carboxylation at a temperature above about 420° C. to form a product with a greatly improved yield of 2,6-naphthalene dicarboxylic acid, or other aromatic dicarboxylic acid.

CROSS REFERENCE

This application claims the benefit of U.S. Provisional Application No.60/151,604, filed Aug. 30, 1999, the entire disclosure of which ishereby incorporated by reference.

This application is related to U.S. Application Ser. Nos. 60/151,577,60/151,607, 60/151,498, 60/151/602, 60/151,603, 60/151,529, 60/151,489,60/151,606, 60/151,589, 60/151,497, 60/151,590, and 60/151,578 filed ofeven date Aug. 30, 1999.

FIELD OF INVENTION

This invention is related to the production of aromatic dicarboxylicacids. More particularly this invention is related to a process forselectively carboxylating an aromatic mono-acid to form primarily anaromatic diacid. One embodiment of the invention is the selectivecarboxylation of naphthoic acid to form primarily 2,3-naphthalenedicarboxylic acid (2,3-NDA). A second part of the invention is theincorporation of the selective carboxylation into a two-stage processfor producing greatly increased yields of aromatic diacid. The inventionmakes greater use of aromatic rings and obtains a surprisingly highyield of an aromatic dicarboxylic acid, such as, for example,2,6-naphthalene dicarboxylic acid (2,6-NDA).

BACKGROUND OF THE INVENTION

Aromatic dicarboxylic acids are highly useful organic compounds. Theyare often used as monomers for the preparation of polymeric materials.For example, terephthalic acid is used to prepare polyethyleneterephthalate, a widely used polyester material and the naphthalenedicarboxylic acids, i. e. 2,6-naphthalene dicarboxylic acid, is aparticularly useful aromatic carboxylic acid because it can be reactedwith ethylene glycol to prepare poly(ethylene-2,6-naphthalate), PEN.Fibers and films manufactured from PEN display improved strength andsuperior thermal properties compared with other polyester materials suchas polyethylene terephthalate. High strength fibers made from PEN can beused to make tire cords, and films made from PEN are advantageously usedto manufacture magnetic recording tape and components for electronicapplications. It is desirable to use as pure as possible forms of thesedicarboxylic acids in the various applications. It is also desirable toobtain as high a yield as possible of the aromatic dicarboxylic acids.

It is known in the art to prepare aromatic dicarboxylic acids byprimarily two methods. One is the liquid phase, metal catalyzedoxidation of an alkyl or acyl substituted aromatic compound. This methodis described, for example, in U.S. Pat. Nos. 2,833,816; 3,856,855;3,870,754; 4,933,491; and 4,950,786. This method has drawbacks. Theprimary disadvantage of the method that involves direct oxidation to 2,6NDA, is that impurities are trapped in the 2,6 NDA oxidation productwhich forms upon oxidation as a solid in the oxidation solvent. In orderto remove these impurities to a sufficiently low level acceptable forpolymerization, the 2,6 NDA product must be purified via multiple steps.These steps typically involve esterification, so that the resulting endproduct is 2,6-naphthalene dicarboxylate, an ester, rather than thepreferred 2,6 napthalene dicarboxylic acid.

Alternatively, naphthalene monocarboxylic acid and naphthalenedicarboxylic acids other than 2,6-naphthalene dicarboxylic acid can beconverted to 2,6-NDA using a disproportionation reaction in the case ofthe monocarboxylic acids or a rearrangement reaction in the case ofother naphthalene dicarboxylic acids. Henkel and Cie first patented areaction of naphthoic acid salts to 2,6 NDA in the late 1950s. (See U.S.Pat. Nos. 2,823,231 and 2,849,482). In these references, it can beobserved that excess base was neutralized out of the feed with HCl withthe objective of having precisely a 1:1 ratio of K:carboxyl. Thesereferences demonstrate the disproportionation of benzene to terephthalicacid+benzene. Isomerization of a diacid such as phthalic to terephthalicwas demonstrated, as well. It can be observed that the best yield ofdiacid in this work was about 65%.

It is known in the art that in normal Henkel disproportionationreactions, a significant yield loss occurs during the reaction. Thisloss, even in the best of circumstances, is usually 3% or more of theweight of the naphthalene dicarboxylic acids (NDAs) theoreticallyexpected to be produced. This loss arises from a mixture of causes, suchas coupling of aromatic radicals to form binaphthyls and highercondensed species, decarboxylation of naphthoic acids to naphthalene,and other undesired reactions.

In the absence of charging other carboxylic acid salts (e.g.tricarboxylic benzene acids, or potassium formates, and the like) thereis no precedent for obtaining a yield of NDA which exceeds thetheoretical yield given by the equation for the Henkel II reaction:

2(potassium naphthoate)→1 napthalene+1 naphthalene dicarboxylic acid,

where the naphthalene dicarboxylic acid is a mixture of isomers, usuallymostly 2,6-naphthalene dicarboxylic acid.

A perplexing question has been how one could more fully use all of therings present in a feed of aromatic monoacids without the need foralkylation or subsequent oxidation. For example, there has not been amethod available in the art to fully use all of the naphthalene ringspresent in a feed of naphthoic acid.

There does not appear to be any work in the art relating to thepossibility of selective carboxylation of monoacids to aromatic diacidsusing inorganic salts. One Japanese reference claims carboxylation inthe presence of oxalates, another organic salt, however only very lowmolar conversion was demonstrated, with only about 2% of the carboxylgroups present in the oxalate being transferred. (cite unavailable)

There is a great demand for dicarboxylic acids in the production ofpolymers, yet it has been difficult to produce dicarboxylic acids ofgood purity and in high yields. It would be a great advance in the artif it were possible to significantly increase the yield of dicarboxylicacids in a disproportionation/isomerization type reaction.

If there were a method available for direct carboxylation of an aromaticmonoacid it would provide a significant advance in the art. It would beparticularly valuable if there were a method for producing the muchsought after 2,6-napthalene dicarboxylic acid in significantly greateryields by direct carboxylation of a feed which is simple to purify andoxidize, such as napthoic acid.

SUMMARY

In the present invention it has been discovered that by operating in anunusual regime of high base and lower temperature, it is possible toproduce a significantly higher ratio of NDA to naphthalene than thetheoretical ratio of 1.0.

In accordance with the foregoing, the present invention comprises amethod of directly carboxylating an aromatic monoacid to an aromaticdiacid, which comprises:

Reacting said aromatic monoacid with excess base in the presence of acatalyst comprising a metal oxide, particularly an oxide of Group IIB,at a temperature of from about 350° to 500°.

A second embodiment of the present invention also comprisessubstantially increasing the yield per pass in adisproportionation/isomerization reaction by a two-stage processcomprising:

Heating overbased naphthoic acid salt at a temperature up to about 420°C. for a relatively short period of time to form 2,3-NDA bycarboxylation, followed by a heating the product for a relatively longerperiod of time at a higher temperature, say above 420° C., to isomerizethe mainly 2,3-NDA product of the first step to 2,6-NDA.

The invention demonstrates an increase in yield per pass in thedisproportionation reaction to form naphthalene dicarboxylic acid, aswell as increased throughput, and the reduced recycling of naphthalene.The present invention more fully utilizes all of the naphthalene ringspresent in a naphthoic acid feed to form naphthalene dicarboxylic acid,without the need for alkylation or subsequent oxidation. The examplesdemonstrate the direct carboxylation of 2-naphthoic acid, and mixturesof 1- and 2-naphthoic.

The present invention makes it possible to have a low capital, highlyefficient disproportionation/isomerization type process to producenaphthalene dicarboxylic acid from naphthoic acid without the need forrecycle of naphthalene for alkylation to naphthoic acid. The presentinvention could greatly simplify and make more productive any processfor producing aromatic dicarboxylic acids, especially those that utilizea disproportionation/isomerization reaction.

DETAILED DESCRIPTION OF THE INVENTION

Disproportionation reactions, such as the Henkel reaction, which areknown in the art, to produce aromatic dicarboxylic acids, particularlynaphthalene dicarboxylic acid, can be represented by the following:

In this reaction, the ratio of base to acid is 1:1. In fact, in earlywork, HCl was employed in the reaction to neutralize all excess base outof the feed. The best yield of 2,6-NDA demonstrated in early work wasabout 65%.

In copending U.S. Application Serial No. 60/151,577 incorporated byreference herein in the entirety, the yield of 2,6-NDA in an integratedprocess incorporating a disproportionation reaction has been optimizedto only about a 3% yield loss.

Carboxylation

The present invention comprises the discovery of a method to selectivelycarboxylate naphthoic acid, or other aromatic mono-acids, to formprimarily 2,3-naphthalene dicarboxylic acid (2,3-NDA) or other aromaticdiacids. This reaction can be represented by:

The process includes the use of an excess of basic carbonates, aspecific temperature range, and salt feeds of particular X-raydiffraction characteristics. In addition, thermogravimetric analysis(TGA) studies of the feeds suitable for the present invention willreveal a relatively low onset temperature of non-drying weight loss onheating (see Table 2). Differential Scanning Calorimetry(DSC) studiesalso show such feeds to exhibit a low melting transition (50° C. or morebelow the normal potassium naphthoate, KNA, melting point of 410° C.)associated with a separate phase.

It is not certain, but it is thought the reaction of the presentinvention proceeds via the formation of a salt bond between the aromaticmonoacid salt and a molecule of bicarbonate or carbonate salt, with thesubsequent formation of an aromatic diacid disalt with the COO(−)M(+),(where M is the metal counter ion), groups adjacent to each other on thearomatic ring (e.g., phthalic acid from benzoic acid, 2,3-NDA from 2-NA,3,4-BDA from 4-carboxy biphenyl, etc.), and a molecule of water andcarbon dioxide (from bicarbonate) or of bicarbonate(from carbonate)containing the hydrogen atom removed from the aromatic ring.

Successful practice of the process of this invention requires sufficientmobility to allow the intermolecular carboxyl transfer to occur via thesalt bridge, and sufficient stability to avoid decarboxylation. Eachsystem requires, therefore, a specific temperature and pressure range tobe effective. However, the key element of the invention is the use ofexcess carbonate, bicarbonate, or related base, which furnishes,ultimately from carbon dioxide gas, the carboxyl groups to betransferred to the ring.

The starting material for the present invention is an aromatic monoacid.Suitable examples include, but are not limited to benzoic, 1-naphthoic,and 2-naphthoic. Alkyl substituted aromatic monoacids will work.

The excess base is a critical element of the present invention. Theoptimum level of overbasing is between 0.1 and 1 moles of excess baseper mole of acid, although this is probably a function of the exactformulation and conditions used. Using potassium carbonate orbicarbonate and naphthoic acid, a suitable range of overbasing is1.05-1.8:1, moles potassium to moles of acid. Good results were obtainedusing 1.2-1.6:1 moles of K per mole of acid.

Suitable bases include alkali metal carbonates. Bases used to providethe excess include, but are not limited to, K₂CO₃, KHCO₃, Rb₂CO₃,RbHCO₃, Cs₂CO₃, CsHCO₃, and other strongly basic carbonates orbicarbonates. The preferred base was potassium carbonate or potassiumbicarbonate.

A suitable catalyst for the carboxylation is an oxide of a metal. Thiscan include a number of metal oxides, but is preferably an oxide of ametal selected from IB, IIB, or IIIA of the Periodic Table, including,but not limited to zinc, cadmium, copper, indium, aluminum, and silver.Good results in the carboxylation of naphthoic acid were obtained usingzinc oxide.

The temperature for carboxylation in the first embodiment of the presentinvention will be in a narrow range. Generally it will be about 50° C.below a suitable temperature for disproportionation/isomerization of thestarting material. The onset of carboxylation for naphthoic acid, forexample, is typically about 380° C., but may be observed in the rangebetween about 375-385° or 390° C. As the temperature is increased,carboxylation takes place up to about 415-425° C., typically about 420°C. Above that temperature potassium salts isomerize anddisproportionation begins to predominate.

Two-Stage Process

The second part of the invention, comprising a two-stage conversion toan aromatic diacid, such as 2,6-naphthalene dicarboxylic acid, consistsof 1.2 to 2.4:1 potassium: naphthoic acid overbased material, processedthrough a low temperature stage (380°-425° C.) to make 2,3-NDA, followedby a high temperature stage (ca.435°-455° C.) to convert the 2,3-NDA to2,6-NDA. The practical overbasing salts will be KHCO₃ and K₂CO₃, and theexact temperature profile and conditions are critical. More base may berequired, depending on the efficiency of utilization of the base in highconversion experiments.

This two-stage process can be effected, inter alia, by feeding overbasednaphthoic acid salt through a heated screw device into a reactor of ahigher temperature for a longer residence time, by feeding a slurry ofsuch naphthoic acid salt into a small vessel or pipeline followed by alarger vessel at a higher temperature, passing hoppers of salt throughfirst a lower and then a higher temperature zone, or other means whichwill be obvious to those skilled in the art to effect the two stagereaction process.

In the two-stage process it is helpful to form a slurry of the feedmaterials. Aromatic hydrocarbons are desired as liquid slurrying media.The slurrying media can suitably be any compound with sufficient thermalstability. It is not restricted to aromatic compounds, however aromaticcompounds are suitable. Examples of suitable solvents include a singlecompound or mixture of compounds selected from a variety of aproticpolycyclic aromatic compounds, such as, for example, naphthalene,methylnaphthalene, dimethylnaphthalene, diphenyl ether, dinaphthylether, terphenyl, anthracene, phenanethrene, and mixtures thereof. Thepreferred medium is naphthalene.

It is demonstrated in the first stage that carboxylation proceeds up to87% on converted naphthoic acid salt under the specified low temperaturereaction conditions. (See Table 1, Ex. 5) In the second stage it isdemonstrated that the 2,3-NDA and other isomers so formed may, byraising the temperature, be converted in at least 80% per pass and 98%overall yield to 2,6-NDA, giving an overall yield of ca. 20% per passand about 95% overall to 2,6-NDA from naphthoic acid via the two stepprocess of this invention. Higher conversions in the first step willincrease the per pass yield of 2,6-NDA to ca. 70%, in excess of anyvalues disclosed in the literature for a single step conversion of2,3-NDA to 2,6-NDA, and more than 40% more than the theoretical yield of2,6-NDA by disproportionation.

So far, 110% vs typical 97% disproportionation yields at 1.2:1overbasing corresponds to about 65% net utilization of the excess basein bicarbonate vs. KOH systems.

The following examples will serve to illustrate specific embodiments ofthe invention disclosed herein. These examples are intended only as ameans of illustration and should not be construed as limiting the scopeof the invention in any way. Those skilled in the art will recognizemany variations that may be made without departing from the spirit ofthe disclosed invention.

EXPERIMENTAL

The examples will demonstrate that by operating in an unusual regime ofhigh base and low temperature, it is possible to produce a significantlyhigher ratio of NDA to naphthalene (NDA/N) than the theoretical ratio of1.000. Without these conditions, the best practical result is about 0.97NDA per Naphthalene (0.97=NDA/N) for the converted K-2-NA salt. Withthese conditions, NDA/N ratios of greater than 1.5 may be obtained. Theinventive process consists of using an excess of base, particularly anexcess of the carbonate and bicarbonate salts formed by CO₂precipitation of 2,6 NDA in the acid form, and holding at lowertemperatures than the conventional Henkel disproportionation (Henkel II)temperatures, most preferably from about 390° C. to about 420° C.Additionally, the most preferred embodiment involves preparation of afinely dispersed mixture of the excess base salts with the naphthoicacid salts, often indicated by low melting peaks in the DSC(differential scanning calorimetry) scans of the feed salts. Further,the reaction may be accelerated by addition of small amounts of largeralkali cation salts (e.g., Cs) which also seem to depress the meltingpoint and improve the mixing of the carbonic and naphthoic salts. In theexamples zinc salts were used to promote the reaction although othersalts may be used, as is known in the literature (Cd, etc.). The zincsalts, preferred for cost and low toxicity, may be used as the oxide,carbonate, or other inorganic salt, or the organic salt of the naphthoicacid feed, conveniently formed by reaction of the naphthoic acid withzinc oxide under elevated temperature.

The experimental procedure comprises adding together naphthoic acid,water, and excess bicarbonate, mixing these starting materials, andheating to form potassium naphthoate, mixed with excess carbonate andbicarbonate. Alternatively, the naphthoic acid is ground up and added toan aqueous solution of K₂CO₃ to form a salt that is soluble in base. Themixture is heated to about 100° C. for about 10 to 30 minutes. Thoseskilled in the art would realize that one could heat at a highertemperature by adding pressure to the water; and one could use a lowertemperature if a longer reaction time is used. Typically, the mixturewas reacted at about 100° C. for about 20 minutes. In addition, particlesize and rate of addition would be affected by the ratio of base tonaphthoic acid. The naphthoic acid is dissolved in base and the waterwith solids is stripped.

The best results have been obtained with a formulation prepared fromnaphthoic acid, an excess of potassium bicarbonate, a specific dryingregimen such that the starting materials exhibit specific x-raydiffraction characteristics, and thermogravimetric analysischaracteristics characterized by a relatively low onset temperature ofnon-drying weight loss on heating (see Table 2). The drying regimeninvolves heating the solids at about 175° C. for about 2 hours under 0.8torr mm Hg pressure. Especially preferred feeds are ones that havemixtures of the following “two theta” peaks (among others) in the powderX-ray diffraction patterns of the initial (gently dried, air exposed)feed salt mixture: 14.0, 28.5, 38.2, 13.6, 27.3, 32.0, and 36.7 degreestwo theta. Not all the peaks need be there in all samples, but ingeneral feeds which have these peaks (as well as others) will give goodyields. These peaks correspond to lattice spacings of 6.32, 3.13, 2.35,6.52, 3.26, 2.80, and 2.45 Angstroms in Bragg d-spacing.

Further description of the experimental procedure includes alternativeways of mixing the starting materials before the drying regimen:

1. The powdered acid can be added to the aqueous material.

2. The acid can be dissolved in hydrocarbon solvent at elevatedtemperature where water with base and catalyst are added and allowed toreact as the water is boiled out of the mixture.

3. The acid and base can be contacted with each other at an elevatedtemperature in the range of about 90-100° C.

4. The water phase or aqueous salts can be dripped into the hydrocarbonsolvent.

5. Powdered naphthoic acid can be dripped into a solution of aqueousbase with catalyst suspended.

In some examples a eutectic mixture was employed. A eutectic mixtureprovides the lowest melting point of a mixture of two or more alkalimetals that is obtainable by varying the percentage of the components.Eutectic mixtures have a definite minimum melting point compared withother combinations of the same metals. For example, though the meltingpoint of LiCO₃ is 622° C., in a eutectic mixture of alkali carbonatesthe melting point can be 400° C. What is required in the presentexamples, where a eutectic mixture is employed, is the right mixture ofalkali metal carbonates where the melting point is less than about400-420° C. Generally the ratio of alkali metal carbonates in theeutectic mixture is about 1:1:1, but it can vary. One eutectic mixtureused as a solvent was K₂CO₃, Rb₂CO₃, Cs₂CO₃, and optionally includingNa₂CO₃.

Best results have been obtained with ZnO as the catalyst forcarboxylation, although a variety of metal oxides will work ascatalysts.

Optimum temperatures for the carboxylation reaction are typically about50° C. below the optimum for a disproportionation/isomerization reactionas known in the art for a similar composition.

Though potassium bicarbonate works well, any “overbased” potassium,rubidium, or cesium carbonate salt, preferably between 1.2:1 alkalimetal:naphthoic acid mole ratio and 2.0:1 alkali metal to naphthoic acidratio appears to make the reaction possible, if run carefully at theoptimum temperature and conditions for the specific material. Apreferred range of overbasing is between 1.2-1.6:1, although this isprobably a function of the exact formulation and conditions used. Morethan half of the excess base present may be transferred to the rings ascarboxyl groups. To fully convert the NA to 2,3-NDA, a full mole ofexcess base is required. In copending U.S. patent application Ser. No.60/151,606, filed of even date and incorporated by reference herein inits entirety, it was shown that overbasing of 0.01 to 0.20 moles permole of monoacid in disproportionation gives optimum 2,6-NDA yield undernormal condition; and that application also showed excellent yields(>95w) of isomerization of the diacid disalts (including K2-2,3-NDA) inthe presence of 0.1 to 0.2 moles of excess base.

Though the art would indicate water should be avoided in a reaction ofthis type, it has been found in the present invention that a smallamount of water, say less than 1000 ppm, seems to actually facilitatethe reaction. It is speculated the beneficial effects are due to theeffect of the water of introducing increased mobility into the salts,specifically allowing the salt complexes more rotational freedom andalso by stabilizing the charged species formed as intermediates. It isalso possible a small amount of water stabilizes the original finelydivided mixed crystalline low melting materials that make the bestfeeds. However, a significant amount of water, say, for example inexcess of ca. 700-1000 ppm, interferes with the reaction by beginning tofavor the decomposition of the salts (decarboxylation). A significantamount of water, 0.2% or more, becomes damaging by promoting yield loss.

In the second embodiment of the present invention, the two-stage processconsists of 1.2 to 2.4:1 K:NA overbased material, processed through alow temperature stage (380°-425° C.) to make 2,3-NDA, followed by a hightemperature stage (ca.435°-455° C.) to convert the 2,3-NDA to 2,6-NDA.The practical overbasing salts will be KHCO₃ and K₂CO₃, and the exacttemperature profile and conditions will be critical. More base may berequired, depending on the efficiency of utilization of the base in highconversion experiments. So far, 110% vs typical 970% disproportionationyields at 1.2:1 overbasing corresponds to about 65% net utilization ofthe excess base in bicarbonate vs. KOH systems. If this efficiency isthe limit, 2.0:1 overbasing would yield 165% of disproportionation and180%(90% direct yield of NA to 2,6-NDA) would require ca. 2.3:1overbasing in the above proposed range.)

Using the two-stage process of this invention, comprising carboxylationof naphthoic acid and subsequent disproportionation and isomerization to2,6-NDA at high conversion per pass of 80% or more, yields of up to 110%of the theoretical yield for a “Henkel” disproportionation have beenobserved at conversions of naphthoic acid up to 90%. At lowerconversions, higher excesses of naphthalene dicarboxylic acid have beenobserved, up to ca. 76% of the converted naphthoic acid.

EXAMPLES 1-6

Examples 1-6 demonstrate the first stage, comprising the selectivecarboxylation of naphthoic acid to 2,3-naphthalene dicarboxylate.Examples 1-6 were carried out using ZnO catalyst and K2NA prepared by1.0 2NA+1.2 KHCO₃. Runs were carried out in a fluidized sand bath usingreactor vessels constructed from ⅜″ diameter stainless steel tubing. Thereactors were charged with 250 psi CO₂ at room temperature.

Yields were measured by proton NMR. In all the runs only a fraction ofthe potassium naphthoate (KNA) was converted to diacid (2,3-isomer),however, of the KNA that was consumed, 0.695 of that mass should bediacid. In the cases of direct carboxylation, the consumed KNA isactually producing more than theoretical amounts of diacid.

Example 6, which was carried out at higher temperature shows typicaldisproportionation/isomerization behavior to give predominantly 2,6-NDAin high selectivity (>60% of NDAs), except that total yield of 2,6-NDAand 2,3-NDA is 110% of theoretical, whereas all lower temperature runsreveal an average K2NA consumption of 25%, little 2,6-isomer, andgreater than theoretical disproportionation yield. Also consistent withdirect carboxylation are the very low naphthalene yields.

TABLE 1 Diacid yield Theoretical Actual Actual Actual Based on ExampleTemp/time K2NA in K2NA 2,3 - isomer 2,6 - isomer Naphthalene ConsumedLR23294 ° C./min NMR tube(mg) Detected(mg) detected (mg) detected (mg)Yield K2NA Ex. 1 410/60 36.7 28.3 8.3 0.6 7% 152% 178-1 Ex. 2 410/6039.29 28.5 9.5 1.3 13%  159% 178-2 Ex. 3 410/30 36.06 27.8 9.0 1.1 9%176% 178-3 Ex. 4 410/15 36.00 29.0 6.4 0.6 11%  144    178-4 Ex. 5410/60 37.66 29.8 9.5 0 8% 174% 191 Ex. 6 450/60 37.01 13.2 5.6 12.797%  111% 192

It should be noted that Example 5 (run 191) represents a utilization of78% of the excess base (K) to produce excess carboxylate. If this ratioholds at higher levels of base proposed above, the proposed theoreticalformulations would yield ca. 90% yields of 2,6-NDA ultimately from NA,or 180% of the theoretical disproportionation yields. Consistent withthe very high level of carboxylation is the very low level ofnaphthalene observed (8% of theory). If this level held to highconversion (conversion in −191 was only 21% of KNA salt, due to lack ofsufficient excess base), one would expect 92% yield of 2,6-NDA overall(184% of disproportionation yield). Run −192 at 65% conversion gaveabout 50% utilization of the excess base. Surplus base is recycled tocontact fresh acid after the removal of the product naphthalenedicarboxylic acid.

EXAMPLE 7

Example 7 was an experiment designed to show thermogravimetric analysis(TGA) reaction temperature onset as a function of overbasing. Theoverbasing ratio is the ratio of potassium in the base used to2-naphthoic acid used to generate the salt. All mixtures contain ca. 17%ZnO.

The onset temperature of reaction by thermogravimetric analysis (TGA) isthe temperature at which non-drying weight loss begins for the givensystem being heated at 10° C./min. under N₂ in this example. It is ameasure of reactivity of the system, influenced by molecular mobilityand carbonate: acid ratio. Naphthalene evolution has been demonstratedfor these systems in a pyroprobe connected to a mass spectrometer,although it is possible in some instances the weight loss could beginwith water from decomposition of bicarbonate formed from thecarboxylation reaction as discussed above. The low temperature peak isabout {fraction (1/10)} the size of the high temperature onset peak(typically 1.5-4% vs 20-30% weight loss). The low temperature peak wouldrepresent water (from carboxylation, drying occurring at much lowertemperature) and the high temperature peak represents naphthalene ofdisproportionation in the simplest model. Some examples also exhibit anintermediate peak.

In this model, the lower temperature is then taken as the minimumtemperature for the carboxylation reaction, and the upper temperature asthe upper limit for carboxylation and lower limit fordisproportionation. It is desirable to operate as near to the upperlimit for carboxylation (higher temp.) as possible to achieve themaximum rate of carboxylation, but preferably below it, to avoidconsumption of naphthoic acid by disproportionation before it can becarboxylated.

KHCO₃ is mostly converted to K₂CO₃ during the 175° C. drying (>75%) andis essentially completely converted by 300° C. Results are shown inTable 2:

TABLE 2 Overbasing Onset of reaction ° C. Onset of reaction ° C. Ratio(a) Base Used Low Temp Peak High Temp Peak 1.0 K₂CO₃ 380 444 1.2 KHCO₃365 414 1.2 K₂CO₃ 250 420 1.4 K₂CO₃ 240 425 2.0 K₂CO₃ 280 420

EXAMPLES 8-19

Examples 8-19 demonstrate isomerization. In Examples 8-19 the followingabbreviations are used: PA=Phthalic Acid; IPA=Isophthalic Acid;TPA=Terephthalic Acid; PAN=Phthalic Anhydride. Examples 8-19 were run at430-460° C., 250 psi CO₂ pressure prior to heating, 3 hours attemperature, no mixing, 150 cc Hoke vessel as reactor, band heater.Hastelloy C vessel, barricaded, for potentially corrosive mixtures,other in s. s. in lab. Final pressures 400-1100 psig, 600-800 psigtypical. Results for the isomerizations of Examples 8-19 are shown inTable 3.

TABLE 3 Bulk Rate D₂O Sol. Feed charged density production Final/initialComponents Products mg/g mg/g* (d) g/cc G/L/hr Solid wts. Comments Ex. 8PA 1.6 165 2.0 — 0.871 430° C. K₂PA −5.27 BA 54.8 — melt 36 K₂SO₄ −4.65Na₂SO₄ −4.79 ZnSO₄ −7.01 Ex. 9 PA 19.7 152 1.8 — 0.834 430° C. K₂PA−3.02 BA 191.4 — melt 115 Li₂CO₃ −1.25 Na₂CO₃ −2.22 K₂CO₃ −1.96 V₂O₅−5.01 Ex. 10 BTA 0.2 500  1.45 0.1 0.6 460° C. run Pan 5.06 TPA 3.6 melt1.7 300 psig H+ Mordenite 5.12 IPA 0.8 0.4 autog. PA 0.6 — CO₂ BA 7.93.8 Ex. 11 PhH 0.3 348 1.2 0.1 0.762 460° C. run PAn 4.66 TPA 106 powder42 (0.853 K₂CO₃ 4.49 PA 4.4 average — of theory) ZnO 4.25 BA 30 12 Ex.12 TPA 172 337 1.2 69 0.664 460° C. PA 4.66 PA 7.8 powder — K₂CO₃ 4.49BA 37.4 avg. 15 Ag₂O 4.68 Ex. 13 BTA 25.8 332 0.7 6.0 0.777 460° C. run.PA 3.12 TPA 130 powder 60.3 Cs₂CO₃ 6.27 IPA 34.9 8.1 PA 6.7 — BA 23.65.5 Ex. 14 BTA 0.3 117 2.2 0.2 0.849 460° C. -172 TPA 0.33 melt 0.24 Pan2.06 IPA 0.57 0.42 Li₂CO₃ 2.64 BA 1.25 0.92 Na₂CO₃ 3.17 K₂CO₃ 2.88Cs₂CO₃ 5.82 Ag₂O 1.0 Ex. 15 BTA 1.46 215 1.4 0.7 0.864 460° C. 23157-23TPA 15.6 powder 7.3 diacids 78% K, Rb, Cs, Zn IPA 3.39 1.6 TPA CO₃eutectic 3.1 PA 1.13 — Al₂O₃ 15.2 BA 13.4 6.2 Pan 3.03 Ex. 16 BTA 1.43131 1.4 0.7 0.888 460° C. -26 TPA 3.59 powder 1.7 BTA, BA K, Rb, Cs, ZnIPA 1.81 0.8 rates CO₃ eutectic 5.03 PA 0.81 — Up/feed Al₂O₃ 15.2 BA8.68 4.1 TPA down Pan 3.03 IPA flat Ex. 17 BTA 0.65 114 1.5 0.3 0.895460° C. -33 TPA 7.70 powder 3.9 84% TPA of K, Rb, Cs, Zn IPA 1.15 0.6diacids CO₃ eutectic 8.06 PA 0.28 — 3 hr run Al₂O₃ 15.57 BA 3.27 1.6 Pan3.05 Ex. 18 BTA 0.40 114 1.5 N/a 0.898 460° C. -30 TPA 0.65 powder 16hrs dup Ex. 17, IPA 0.92 except PA 0.45 went 16 hrs BA 2.58 Ex. 19 BA0.78 143 2.0 1.3 0.879 460° C. -31 melt K₂PA 5.26 Cs₂O₄ 7.99 K₂SO₄ 4.71ZnSO₄ 7.07

The analytical procedure was as follows: The entire solid sample washomogenized by grinding, and one gram was digested with 10 cc of D₂Ospiked with trimethylsilyl sodium propionate(TSP) for 4 hours at 70° C.One cc of solution was then analyzed by H-nmr, and values of identifiedspecies (PA, IPA, TPA, BA, etc.) calculated vs. TSP standard andreported. Multiplication of mg observed in sample×10 yields mg/g oforiginal solid from reactor.

If the feed was charged as a solid (e.g. PAN), it was mixed with theother materials as powders and presumed to be uniformly distributed. Insuch cases the mg/g is simply the fraction by weight of materialcharged. If the feed charged is a liquid under the reaction conditions,it was treated as a solid, above.

Estimated bulk density for the mixture under reaction conditions may notbe the same as the components at room temperature. It is assumed thatthe molten salt species have a density of 0.68 of that of the pure solidphases when molten. The diacid salts are assumed not to melt (basisexperiments with TPA salts) and are taken at their solid phasedensities. If the system is not molten (i.e., recovered as a powder),the phase densities are divided by 3 to get an approximation of the bulkdensity. The idea is to estimate bulk density under reactionconditions.(g/cc)

Calculated rate of production of specified component is based on theobserved production per gram of solid, the bulk density, and the time.(g/liter/hour)

Ratio of weight of material charged to weight recovered; some losses arenormal due to adhesion to walls, funnels, scrapers, etc. andquantitative recovery is not expected—but gross discrepancies indicatevolatilization, loss of CO₂, etc.

Feed per g calculated per unit volume of 1 g of solid.

EXAMPLES 20-25

Examples 20-25 relate specifically to the production of naphthalenedicarboxylic acid (NDA) and further demonstrate that operating in theregime of the present invention of high base and low temperature, it ispossible to produce a significantly higher ratio of NDA to Naphthalene(NDA/N) than the theoretical ratio of 1.000. Without these conditions,the best practical result is about 0.97 NDA per Naphthalene (0.97=NDA/N)for the converted K-2-NA salt. With these conditions, NDA/N ratios ofgreater than 1.5 may be obtained. The inventive process consists ofusing an excess of base, particularly an excess of the carbonate andbicarbonate salts formed by CO₂ precipitation of 2,6 NDA in the acidform, and holding at lower temperatures than the conventional Henkeldisproportionation (Henkel II) temperatures, most preferably from about390° C. to about 420° C. Additionally, the most preferred embodimentinvolves preparation of a finely dispersed mixture of the excess basesalts with the naphthoic acid salts, often indicated by low meltingpeaks in the DSC (differential scanning calorimetry) scans of the feedsalts. Further, the reaction may be accelerated by addition of smallamounts of larger alkali cation salts (e.g., Cs) which also seem todepress the melting point and improve the mixing of the carbonic andnaphthoic salts. In the examples zinc salts were used to promote thereaction although other salts may be used, as is known in the literature(Cd, etc.). The zinc salts, preferred for cost and low toxicity, may beused as the oxide, carbonate, or other inorganic salt, or the organicsalt of the naphthoic acid feed, conveniently formed by reaction of thenaphthoic acid with zinc oxide under elevated temperature.

A convenient way of forming the preferred disordered salt mixture is tocharge a mixture of the organic naphthoic acid potassium (or mixedalkali) salt in water with dissolved inorganic (carbonate, bicarbonate)salts and suspended zinc oxide, into a vessel of hot oil, preferablynaphthalene or another stable hydrocarbon consistent with the process.By this means, the solution is rapidly converted to a porous solid ofcomparatively fine crystallite size and intimately mixed organic andinorganic salts. In order to analyze precisely for the amount ofproducts produced, three repeated extractions of the salt phase with KOHin D₂O and a suitable protonic internal standard for nmr were made toanalyze for the acids by quantitative nuclear magnetic resonance, andthe hydrocarbon (naphthalene) portion of the product was analyzed bydigesting the sample in d6 DMSO(deuterio dimethyl sulfoxide) withtrioxane as the internal protonic standard. By using test synthetic saltmixtures of K2-2,3 NDA, K2-2,6 NDA, K-2-NA, and naphthalene, it wasshown that the error in the analysis by this method was less than 1%mole of the contained species analyzed.

EXAMPLE 20

Example 20 describes the preparation of feed salt. The feed salt samplesheated to induce disproportionation were first prepared as mixed saltsby stripping from aqueous solution, including 5% by weight of dried ZnOpowder, drying at 125° C. in a vacuum oven, and were then ground in amicromill to produce a fine (5-10 micron) powder. The powder was thendried at 175° C. in a vacuum oven at 1-2 torr pressure. The dry powderat 175° C. was conveyed quickly through the atmosphere and charged intoa 100 cc autoclave heated to 130° C. to ensure dryness, sealed, andpurged repeatedly with CO₂ to remove most of the air and provide the gascap for the disproportionation reaction. The autoclave was then fittedonto a “rotisserie” rack in a Blue M convection furnace designed tomaintain temperature at about +/−2° C. and heated for the required time.Heat up and cool down in this oven require about 15 and 20 minutesrespectively. The oven is purged with nitrogen in the event a leakreleases potentially flammable naphthalene over the heater in the aircirculation pathway.

EXAMPLE 21 Comparative

In the comparative example, 5.0 g of mixed salts containing 1.00 to1.1:1 K:2-NA ratio are prepared as described with 0.25 g of ZnO, chargedinto the autoclaves as described and heated at 450° C. for 1.5 hours.The resulting NDA/N ratio of the product is between 1.00 and 0.95depending on the exact preparation (amount of carbonate, bicarbonate,crystal size, DSC trace, XRD pattern of product salt, uniformity ofmixing, etc.) Conversion of 2-NA to products is from 80% to 99%depending on CO₂ pressure and exact conditions as described above.Selectivity to 2,6 NDA in the NDA product is more than 70%.

EXAMPLE 22

In Example 22, a ratio of inorganic (carbonate, bicarbonate) basic saltsto 2-naphthoic acid of 1.3:1 is used to prepare 5.0 g of mixed salts.The resulting salt is prepared as described with 0.25 g of ZnO, andadded as described to the autoclaves and heated for 1.5 hours with a 20minute hold period at 420° C. and 70 minutes at 450° C. (includingtransient heating to the higher temperature. The resulting NDA to Nratio is an average of 1.15 in this case, indicating a surplus of up toca. 20% of NDA. In this example, an average conversion of 2-NA salt of94% is observed, with an average selectivity to 2,6 NDA of 88% of thetotal NDAs formed. As usual, the major other NDA salt formed is the 2,3isomer.

EXAMPLE 23

In Example 23, the mixture of example 22 is heated to 450° C. directly.An NDA/N ratio of 1.03 to 1.14 is observed, indicating thatcarboxylation of the naphthoic acid may occur during the temperatures ofconventional Henkel isomerization and the normal heat-up period,provided sufficient excess base is present.

EXAMPLE 24

In Example 24, a variety of K:2-NA ratios is used, from 1.2:1 to 2.0:1,with the slow heat up cycle of Example 22. The resulting NDA yieldsbased on 2-NA conversion range up to 40% excess of theoretical yieldfrom the disproportionation reaction. 2,3 NDA is the major non-2,6 NDAisomer formed, but again 2,6 NDA is typically about 80-90% of the totalNDAs. An optimum appears around 1.3 to 1.6 to 1 K:2-NA ratio, probablydue to an optimal level of mixing in the combined organic/inorganic(naphthoate/carbonate&bicarbonate) salts and dispersion of the catalyst.Additionally, it is thought that ratios of 2.0 to 1 or less arepreferred strictly on yield grounds due the reduced level of naphthoateper unit volume in the high ratio materials. In addition, it is moredifficult to recycle the salts at high ratios, due to reducedeffectiveness of the preferred CO₂ preciptation of 2,6 NDA at high baselevels. Therefore it is apparent that for a given process configuration,there will be an optimum level of excess base, an optimum temperatureprofile and residence time, etc

EXAMPLE 25

In Example 25, the experiment of Example 22 is repeated withsubstitution of 9% of the potassium with Cs. It is observed that the NDAto N ratio is about 1.1:1. However, if the temperature is lowered to430° C. it is also observed that the kinetics are substantially faster,with a greater rate of production of NDAs at 430° C. than at 450° C. inthe pure potassium case. It is thought that the Cs/K mixture favors thedisordered salt phase preferred for the carboxylation anddisproportionation and isomerization reactions. It is further observedthat eutectic mixtures of 1- and 2-napthoic acids as a mixed salt of Cs,K, and Rb may be formed over a fairly wide ratio of alkali ions andorganic isomers. Such eutectics may melt as low as about 300° C., andgive a rapid disproportionation and isomerization as well ascarboxylation as low as 380° C. However, they are not generally the mostpreferred embodiment, due to the cost of the heavier alkali ions and thedifficulties of separation in the CO₂ precipitation phase (oiling out ofsalts etc.).

We claim:
 1. A two step process for increasing the yield of a desiredaromatic dicarboxylic acid which comprises: a) selectively carboxylatingan aromatic mono-acid by the steps of: mixing an aromatic monoacid withexcess base to form a solid salt; drying said salt; and reacting saidsalt in the presence of a catalyst selected from an oxide of a metal ofGroup IB, IIB, or IIIA of the Periodic Table at a temperature aboveabout 350° C. to produce an intermediate containing an isomer of thedesired aromatic dicarboxylic acid, the isomer containing a carboxylgroup contributed by the base; and subsequently b) reacting saidintermediate in the presence of a disproportionation catalyst at atemperature above about 420° C. to form the desired aromaticdicarboxylic acid.
 2. The process of claim 1 wherein the aromaticmono-acid is selected from the group consisting of benzoic acid,1-naphthoic acid, and 2-naphthoic acid.
 3. The process of claim 1wherein the base is an alkali carbonate selected from the groupconsisting of alkali metal carbonates and bicarbonates.
 4. The processof claim 3 wherein the base is selected from the group consisting ofK₂CO₃, KHCO₃, Rb₂CO₃, RbHCO₃, Cs₂CO₃, and CsHCO₃.
 5. The process ofclaim 1 wherein the temperature for reacting said salt in the presenceof a catalyst is from about 380° C. to about 420° C.
 6. The process ofclaim 1 wherein the catalyst for the first stage is an oxide of a metalselected from Group IB, IIB, or IIIA of the Periodic Table.
 7. Theprocess of claim 6 wherein the metal is selected from the groupconsisting of oxides of zinc, cadmium, copper, indium, aluminum, andsilver.
 8. The process of claim 1 further comprising the presence of upto 1000 ppm water in the reaction.
 9. The process of claim 1 whereinstep b takes place at a temperature of about 430-480° C.
 10. The processof claim 1 wherein said solid salts are dried by heating the solids atabout 175° C. for about 2 hours under 0.8 torr mm Hg pressure.